The present invention relates generally to circuitry and methods for providing constant current to a light source such as an LED load. More particularly, the present invention relates to an adaptive startup voltage control for a constant current LED driver.
Light emitting diode (“LED”) lighting is growing in popularity due to decreasing costs and long life compared to incandescent lighting and fluorescent lighting. LED lighting can also be dimmed without impairing the useful life of the LED light source, and is generally effective in extremely cold environments, such as for example −40 degrees Celsius (° C.).
Generally speaking, LED lighting applications are designed to control the output current in the load and not the voltage, as the forward voltage of an LED load can vary dramatically, for example as a function of temperature, and a fixed voltage output would likewise produce varying output current and lighting output levels. Conventional LED drivers therefore sense an output current provided to the load and a feedback circuit implemented by the controller maintains the output current to a substantially constant value.
However, as the current flow through the LED load may vary as a function of temperature, the brightness of the LED lighting output may change along with the temperature of the LED load even while the current supplied from the LED driver remains substantially constant. Therefore, at least for certain temperature variations, the output current from the driver circuit to the LED load must vary as a function of temperature in order to maintain appropriately constant lighting output.
A typical representation of forward voltage drop characteristics for an LED load versus temperature is shown in FIG. 1. As represented in FIG. 1, the incremental forward voltage drop for an LED load is relatively flat, or in other words substantially unchanging, after the temperature of the load exceeds about 20 degrees Celsius. However, the load voltage may be dramatically higher at −40 degrees Celsius than a voltage for the same LED load at a temperature such as 20 degrees Celsius, and typically it could approach levels 30% higher or more.
This forward drop voltage differential for loads at such different temperatures is a challenge for LED driver designers. When an LED load reaches steady state operation at full bright lighting output, it is generally hot enough to maintain a reasonably high temperature (for example >20 degrees Celsius) because LEDs generate heat after a relatively short warm up time. LED driver designs typically include a maximum output voltage to limit the output power of the driver. This maximum output voltage limit typically is designed around the forward voltage drop at the steady state temperature (e.g., around 20° C.).
However, again with reference to the characteristic curve in FIG. 1, if the temperature at startup is around −40° C. the forward voltage of the LED load will be much greater than the voltage at 20° C. The driver circuit might not be able to start the LED load at −40° C. or otherwise maintain full current if the driver output voltage limit is set around Vf+20 (i.e., forward drop at +20° C.). If the LED driver design is accordingly modified to force the maximum output voltage limit to be the forward drop at −40° C., the LED driver output voltage will by necessity be over-designed for steady state operation, with a power limit substantially 30% higher or even more with respect to a desired configuration. This over-voltage design is highly undesirable, as it would typically increase the product cost and size dramatically.